This disclosure relates to a system and process of generating synthesis gas in a gas turbine combined cycle.
In many industrialized nations, power plants are subject to, or will become subject to, economic penalties for emission of various materials to the environment. For example, controls on carbon dioxide emissions results in emissions trading and taxes, and requires permits. As the cost of emission penalties increase, measures to curb emissions are continually sought, either as a retrofit to existing plants to reduce emissions and/or as a design of new carbon dioxide (CO2)-lean power plants to lower overall emissions.
One CO2 capture method is a fuel decarbonisation process, also known as a pre-combustion capture pathway. The process converts hydrocarbon (HC) fuels to a hydrogen-based fuel and carbon monoxide (e.g., synthesis gas or “syngas”). The carbon monoxide (CO) is then converted to carbon dioxide, which is removed from the system before the combustion process in the power plant.
A typical decarbonisation plant is highly complex as it involves many catalytic reactors such as a desulphurization reactor, pre-reformer, auto-thermal reformer and water-gas-shift reactor. Furthermore, the process is currently thermodynamically inefficient and costly. For example, a typical decarbonisation process often results in an 8-12% penalty in the overall plant efficiency due to the energy required and released during the reforming process. Thus, there is a strong need to create a more efficient decarbonisation system.
Traditionally, heat recovery in decarbonisation processes have involved generating low and high-pressure steam depending on the temperature level of the heat available. Steam has been preferred in the past because it is well known and widely used in the industry; however, alone, it is ineffective at significantly improving the efficiency of the process. Although energy is released during reformation at a temperature greater than about 800° C., this heat is recovered in the form of saturated steam at only about 200° C. to about 300° C. Saturated steam is generated to avoid metal temperatures in the region of about 500° C. to about 900° C.; which can result in metal dusting. This results in a large quantity of unused energy and large efficiency penalties. The steam is superheated in the exhaust gas of the combined cycle and low temperature heat is often rejected to either the environment or the low-pressure section of the steam bottoming cycle.
In order to meet emission goals, a power plant can employ a fuel decarbonisation system designed to minimize CO2 emissions. These systems reform a hydrocarbon fuel (for example, natural gas) into a synthesis gas, comprising CO and H2. Unfortunately, the reformation process results in a large penalty to the overall efficiency of a power plant primarily due to the loss of high temperature energy. Furthermore, low temperature heat may be rejected either to the environment or to a low-pressure section of a steam bottoming cycle. Therefore, a more efficient decarbonisation process is needed, which captures the high temperature heat energy released during reformation and utilizes the low temperature heat energy rejected throughout the cycle.